7. Project Summary/Abstract My research is rooted in a developmental psychopathology framework and focuses on understanding mechanisms that influence the development and persistence of aggression in youth, with the long-term goal of translating this knowledge to develop more effective prevention and intervention efforts. The prevalence of aggression among clinically referred youth and the well-documented associated risks speaks to the urgent need for research to identify child and familial risk mechanisms to better determine who is most vulnerable and how to best intervene with those youth and their families. The current proposal seeks to examine emotional reactivity as a neurobiological vulnerability for the persistence of reactive aggression (RA) during the critical transition to adolescence, a time period marked by substantial changes in the neurobiology underlying emotional development. Additionally, it aims to assess how dynamic parent-child transactions may serve to exacerbate RA over time, as parenting may be particularly potent for at-risk youth during this developmentally sensitive period. This application details a comprehensive training plan that allows me to address these questions and build on my previous experiences in several important ways. First, in light of the heterogeneity among youth engaging in aggressive behaviors, I aim to examine mechanisms underlying a more specific behavioral phenotype of aggression, namely RA, during the transition to adolescence, a time of rapid neurobiological change. Second, I seek to expand my knowledge of neurobiological assessment modalities to psychophysiological assessment of mechanisms underlying RA. Third, I hope to master the processing and analysis of ecological momentary assessments as an innovative tool to comprehensively assess RA and its underlying mechanisms in daily life. Concordant with NIMH strategic priorities 2.1 and 2.2, the current proposal takes a transactional approach and focuses on within-individual differences by utilizing a multi-modal, ecologically informed assessment of RA, emotional reactivity and parenting during the transition to adolescence. Independent and interactive effects of individual variation in emotional reactivity and parenting dimensions on the development and persistence of RA over 18 months will be tested in a clinically at-risk sample of youth. The aims of the study are 1) investigate concurrent and prospective associations between individual variation in emotional reactivity and RA during the transition to adolescence; 2) examine the bi- directional nature of parental response to emotional reactivity and RA over 18 months; 3) assess parental response to emotional reactivity as a moderator of the association between emotional reactivity and RA. The central hypothesis of this proposal is that heightened emotional reactivity as measured multidimensionally confers risk for the persistence of RA into adolescence, with variation in parental response to emotional reactivity and RA serving to exacerbate or ameliorate this association. This knowledge can ultimately be used to further develop prevention and intervention targets for these specific processes.